While reading the original source code, it seems that Py_TPFLAGS_INLINE_VALUES was intended to be (1 << 2), based on how the flag is used.
So a simple fix might be to change it to:

Also, Py_TPFLAGS_MANAGED_DICT is used in the TestInlineValues tests.

Let’s see what Mark and Ken Jin think about this.

The best way is to find a class that has the Py_TPFLAGS_MANAGED_DICT and Py_TPFLAGS_INLINE_VALUES mutual exclusion case.

what if we add a case that specifically uses a subclass of a variable-sized type, like tuple?

1. It gets the Py_TPFLAGS_MANAGED_DICT flag, as it's a heap type that receives a __dict__.
2. It does not get the Py_TPFLAGS_INLINE_VALUES flag because it inherits a non-zero tp_itemsize from tuple, and the INLINE_VALUES optimization is only applied to fixed-size objects.

Adding the following test would explicitly verify this intended behavior and help protect against future regressions.

class VarSizedSubclass(tuple):
    """
    Mutual exclusion case: A class inheriting from a variable-sized
    built-in type. It has a managed dict, but no inline values.
    """
    pass

class TestTypeFlags(unittest.TestCase):

    def test_flags_for_mutual_exclusion_case(self):
        """
        Tests the mutual exclusion case: VarSizedSubclass should only
        have the MANAGED_DICT flag.
        """
        flags = VarSizedSubclass.__flags__
        
        # 1. It should have the MANAGED_DICT flag.
        self.assertEqual(flags & Py_TPFLAGS_MANAGED_DICT, Py_TPFLAGS_MANAGED_DICT)
        
        # 2. It should NOT have the INLINE_VALUES flag.
        self.assertEqual(flags & Py_TPFLAGS_INLINE_VALUES, 0)

What do you think of this approach?

Yeah looks good to me, just please integrate them into TestInlineValues rather than add new extra test.

I've updated the tests as discussed, following the approach in 9cd9c5b (#136538)

I've added the new test cases for the various flag combinations to the existing TestInlineValues class. I'm a little hesitant about whether this is the most appropriate place, as the name TestInlineValues might now be a bit narrow for the expanded scope of these tests.

Do you think it's acceptable to expand the scope of this class, or would you prefer a different approach (like renaming the class to TestTypeFlags, for example)?